Polychlorinated hydrocarbons, such as carbon tetrachloride, chloroform, trichloroethylene, and tetrachloroethylene have been widely used as chemical intermediates, solvents for dry cleaning of clothing, degreasing, and a variety of other applications. Halogenated hydrocarbons are very stable and persistent in the environment. They are relatively toxic at low levels.
Over the years, halogenated hydrocarbons have been accumulating in the environment, particularly in groundwaters. As a result, pollution of water by chlorinated hydrocarbons has become an important environmental problem and contaminated groundwaters represent a large portion of environmental remedial action plans throughout the world.
It is known that chlorinated compounds can be degraded by reductive dechlorination, that is, replacement of chlorine substituents by hydrogen. Evidence indicates that many of the chlorinated solvents are degraded under anaerobic conditions in sediments and in systems simulating anaerobic wastewater treatment. Current theories suggest that some of these anaerobic dechlorinations are not strictly biological in nature, but are catalyzed by metallic iron.
In the patent literature, patents are issued that use metals or metallic couples to degrade chlorinated organic compounds. In U.S. Pat. No. 3,640,821 to K. H. Sweeney and J. R. Fischer, metallic zinc is used for removing pesticides from aqueous solutions. U.S. Pat. No. 3,737,384, also to Sweeney and Fischer, discloses the use of metallic couples, in solutions buffered to near neutral pH for the degradation of pesticides.
More recently, researchers in Japan have reported on the degradation of 1,1,2,2-tetrachloroethane and trichloroethylene in aqueous solution in the presence of iron powder: Senzaki, T. and Y. Kumagai, "Removal of Chlorinated Organic Compounds from Wastewater by Reduction Process: II. Treatment of Trichloroethylene with Iron Powder" Kogyo Yosui, 1989, 369, 19-25. Gillham and O'Hannesin in their article "Metal-Catalyzed Abiotic Degradation of Halogenated Organic Compounds" IAH Conference on Modern Trends in Hydrogeology: Hamilton, Ontario, May 10-13, 1992, have confirmed Senzaki's results. Recently, Gillham received a U.S. Pat. No. 5,266,213, for his method for cleaning halogenated contaminants from groundwater. The process involves feeding contaminated groundwater through a trench containing a metal such as iron, under strict exclusion of oxygen, and over a lengthy period of time.
While the above-mentioned metal systems show the reductive dechlorination of hydrocarbons in aqueous solutions in the presence of iron, they are disadvantaged in that large amounts of iron metal are needed for completion of the reactions, as well as, substantial periods of reaction time are required.
There is a need for an enhanced method for dechlorinating hydrocarbons in contaminated aqueous solutions so as to diminish the amount of metallic iron needed to complete the reaction. There is also a need for a dechlorination method where the rate of the reaction is increased, the amount of metal iron as a reductant is decreased, and innocuous reaction products are generated.